Gravitational Waves from Hyper-Accretion onto Nascent Black Holes

Abstract

We examine the possibility that hyper-accretion onto newly born, black holes occurs in highly intermittent, non-asymmetric fashion favorable to gravitational wave emission in a neutrino cooled disk. This picture of near-hole accretion is motivated by magneto-rotationally induced, ultra-relativistic disk dynamics in the region of the flow bounded from below by the marginally bound geodesic radius. For high spin values, a largely coherent magnetic field in this region has the dynamical implication of compact mass segregation at the displacement nodes of the non-axisymmetric, MRI modes. When neutrino stress competes favorably for the disk dynamical structure, the matter clumps may be rather dense and sufficiently long-lived to excite the Quasi-Normal Ringing (a.k.a. QNR) modes of the Kerr geometry upon their in-fall. We find that such accretion flow may drive bar-like, quadrupole (l,m=2,2) modes in nearly resonant fashion for spin parameters $a \geq .9$. The ensuing build up in strain amplitude of the undamped oscillations warrants a brisk rate of energy deposition into gravitational waves. A detectability assessment for the LIGO interferometers through the match filtering technique is given by integrating the energy flux over a one second epoch of resonant hyper-accretion at $1 \msun \sec^{-1}$. Thus, a $15 \msun$ Kerr black hole spinning at $a \simeq .98$ ($f_{\rm QNR} \simeq 1677$ Hz), and located at 27 Mpc (e.g., GRB980425), will deliver a characteristic strain amplitude, $h_{\rm char} \simeq 2.2_{-21}$, large enough to be detectable by LIGO II. If resonant hyper-accretion were sustainable for a longer period (or at higher rates) possibly associated with a second broad hump in a GRB light-curve, these objects could be detected by LIGO I at very low redshifts.